Drill feed control

ABSTRACT

Feed control comprises a frame attached to a drill motor and carrying an axially slidable guide shaft with a pressure foot at forward end to contact a work piece to be drilled. A lead screw is carried by the frame and rotatable about an axis parallel to the axis of the drill motor and drill bit. A nut member carried by the guide shaft engages the lead screw and prevents retraction of the guide shaft and relative advance of drill bit toward and into the work piece when axial force is applied. A drive train connected to the lead screw and the drive shaft rotates the former when the latter is rotated, and enables the nut member to &#39;&#39;&#39;&#39;back off&#39;&#39;&#39;&#39; along the lead screw so that the guide shaft retracts gradually at a predetermined rate and the bit advances at a corresponding rate. Since the bit cannot advance freely and independently, it cannot surge or jam when it breaks through the rear of the work piece. In preferred form, a clutch is provided in the drive train and actuated by load on pressure foot, which prevents backing off when not actually drilling. Device works in same way when applied to drill press or power feed machines mounted on fixtures or work pieces.

United States Patent [1 1 Adams [11] 3,804,544 1 Apr. 16, 1974 DRILLFEED CONTROL Ralph D. Adams, 1210 Portola Ave., Escondido, Calif. 92025Filed: Dec. 29, 1971 Appl. No.: 213,683

Inventor:

References Cited UNITED STATES PATENTS 7/1972 Bohorquez 408/14 f3/l957Quackenbush 9/1933 Park et al. 408/110 X Primary Examiner-Gil WeidenfeldAttorney Agent, or Firm-F orrest J. Lilly [57] 7 ABSTRACT Feed. controlcomprises a frame attached to a drill motor and carrying an axiallyslidable guide shaft with a pressure foot at forward end to contact awork piece to be drilled. A lead screw is carried by the frame androtatable about an axis parallel to the axis of the drill motor anddrill bit. A nut member carried by the guide shaft engages the leadscrew and prevents retraction of the guide shaft and relative advance ofdrill bit toward and into the work piece when axial force is applied. Adrive train connected to the lead screw and the drive shaft rotates theformer when the latter is r'otated, and enables the nut member to backoff" along the lead screw so that the guide shaft retracts gradually ata predetermined rate and the bit advances at a corresponding rate. Sincethe bit cannot advance freely and independently, it cannot surge or jamwhen it breaks through the rear of the work piece. In preferred form, aclutch is provided in the drive train and actuated byload on pressurefoot, which prevents backing off when not actually drilling. Deviceworks in same way when applied to drill'press or power feed machinesmounted on fixtures or work pieces.

"23 Claims, 7 Drawing Figures 1 DRILL FEED CONTROL BACKGROUND OF THEINVENTION This invention lies, in the field of controls for the rate ofadvance or feed of drills into work pieces, and is directed particularlyto apparatus which prevents the feeding of drills at excessive rates andalso prevents surging or jamming of drills as they break through therear faces of work pieces.

It is well known that there are optimum rates of axial feeds of drillsfor various sizes and rotational speeds and for various materials. Whendrills are used in continual production work it is normal to select theproper tip shape and rotational speed for a given size bit working in agiven material in order to drill cleanly and rap idly with minimumdulling of the cutting edges. However, in the case of hand held drillsand manually controlled drill presses, the amount of axial force appliedand the rate of feedof the drill bit into the work are actuallydetermined by the experience and judgment of the individual operator.This is not a simple matter, and the results vary widely.

In addition, a great deal of such work is performed on work piecescomprising one or more sheets of relatively thin metal, particularly inthe aerospace industry. Such sheets are somewhat elastic and normallyare not supported immediately adjacent'to the point of drilling, withthe result that they are elastically displaced by the drill pressure.When the drill breaks through the rear, the sheets spring backelastically and may jam or break the drill as well as produce burrs.Another problem with drilling multiple layers is that when the drillbreaks through one layer or sheet, that sheet frequently springs awayfrom the next one and leaves a gap in which burrs may lodge, usuallywith no possibility of removing them.

The same general problems are encountered with manually controlled oroperated drill presses because the basic operation is the same with thepress stand supporting the drill for the man. In either case thedrilling operation is slowed down because the man starts toautomatically hold back as he feels that he is approaching the'end ofthe cut in order to avoid surge, jamming, or breaking of the drill.

Power feeddrilli n'g machines also have these problems because basicallythey are merely substituting external power for manpower and havenomeans for sensing the final stage of the cut. 4

SUMMARY OF THE INVENTION,

The present invention overcomes the difficulties mentioned above andoffers a device which is simple in construction and operation as it actsto positively limit the maximum rate of axial feed to a predeterminedamount and requires maintenance, of adequate pressure on the work pieceduring the drilling operation.

Generally stated, in preferred form, the basic unit includes a framewhich is attached to the drill motor and which carries a guide shaftwhich is slidably mounted for axial movement parallel to the axis of thedrill motor or drill bit in advancing and retracting directions withrespect to the bit. In a first form, the aft portion of the shaft isslidable on the frame and the forward free end portion extends beyondthe frame and carries a pressure foot which has a pressure pad forengaging the work piece adjacent to the point of drilling. Re-

straining means is built into the frame and includes a lead screwmounted for rotation about its axis parallel to the drill axis and withonly slight axial movement, and a nut member carried by the guide shaftand engag: ing the threads on the lead screw. Ordinarily the guide shaftis initially adjusted to such a forward or advanced position that whenthe pressure foot is in contact with the work piece the tip of the drillbit is just in contact or slightly spaced rearward. Pressure applied tothe drill motor and thence to the pressure foot results in back pressureon the guide shaft which is positively opposed by the restraining means,and the drill cannot penetrate the work piece.

A drive train, preferably a plurality of reduction gears, is connectedbetween the drill motor drive shaft and the lead screw so that when thedrive shaft is rotated the lead screw is also rotated at a predeterminedlower rate. .The nut member backs off on the lead screw and allows theguide shaft to retract, in effect advancing the drill bit to or throughthe work piece. The rate of feed is predetermined by the gear reductionand cannot be exceeded regardless of the axial pressure on the drillmotor or drive shaft. Since the pressure is always directly applied tothe first surface of the work piece there is no spring-back or surgewhen the drill tip reaches the last surface and there is nointer-laminar gap to receive burrs.

The thread form on the lead screw is basically a buttress threadarranged with its high angle face opposing the thrust of the nut member.The latter is resiliently biased into engagement and is readilydisplacedlaterally against the bias by-the low angle faces so that the guideshaft and nut member may be advanced rapidly for a new drillingoperation.

If the drill motor is actuated only when the pressure foot is in firmcontact with the work piece, the lead screw may be positively connectedwith the drive shaft. However, if it is actuated without drilling, thelead screw will be rotated and will build up slack so that the guideshaft may retract and not perform its function. Hence, in the preferredform, a normally disengaged clutch is incorporated in the end of thedrive train adjacent to the lead screw, which is mounted for a smallamount of axial movement. If no drilling is being done, actuation of thedrill motor will not actuate the lead screw. However, when the pressurefoot is in contact with the work piece and adequate pressure is applied,the guide shaft, nut member, and lead screw are retracted enough tocause the clutch members to engage and operate the lead screw to producethe proper feed rate. A spring is preferably incorporated to maintainthe clutch members disengaged in the absence of drilling pressure.

The same construction can be applied to power feed machines mounted ondrilling fixtures or clamped to work pieces and will operate in the sameway.

The same construction may also be used with conventional stationarydrill presses. However, a desirable modification in this case comprisesextending the guide shaft aft instead of forward and providing a contactmember on the guide shaft to engage an ear on the spindle housing whichwill serve as a reference member instead of the first surface of thework piece since it is at a constant distance from the latter. 7

BRIEF DESCRIPTION OF THE DRAWINGS Various other advantages and featuresof novelty will become apparent as the description proceeds inconjunction with the accompanying drawings, in which:

' FIG. 1 is a schematic view in perspective of a typical I hand helddrill motor with the feed control of the invention attached thereto;

FIG. 2 is a schematic side elevational view, partly in section, of thebasic arrangement of the components;

DESCRIPTION OF PREFERRED EMBODIMENTS The general arrangement illustratedin FIG. 1 shows a typical electric or air operated drill motor having adrive shaft 12 and a standard chuck 14 mounting a drill bit 16. A frame18 secured to motor 10 by bracket 19 is preferably a casing to enclosevarious components and carries an extension drive shaft 28, (FIG. 2),which is interposed between drive shaft 12 and chuck 14. A guide shaft20 is slidably mounted in the forward part of the frame and is fixedlyconnected at its forward end to pressure foot 22 having at one end apressure pad 24 to engage a work piece adjacent to the point ofdrilling. The pressure pad is shown as a yoke but it may be an annulus,and in some cases may be provided with a drill bushing.

As illustrated in FIG. 2, the frame 18 may comprise a casing 26 made upof a plurality of separable parts, not indicated, for proper assemblyand support of various components. The upper part of the casing enclosesan extension drive shaft 28 having an integral drive pinion 30. The aftend of the shaft is connected to the motor shaft 12 and the forwardportion passes through a double row ball bearing 32 mounted in thecasing or frame, the forward end 34 extending out of the casing toreceive chuck 14.

The aft portion 36 of guide shaft 20 is axially slidably mounted in apassage 38 of the frame for advancing and retracting movement withrespectto drill bit 16 in a path parallel to the axes of the drill motorand the bit, and pressure pad 24 is located to contact the first surfaceof the work piece 40 adjacent to the point of drilling. The work piecemay be one or more pieces of thick or thin material.

A lead screw 42, rotatable about its own axis parallel to the axis ofthe drill motor, has a forward shank 44 mounted in bearing 46 and an aftshank 48 rotatably mounted in sleeve 50, which in turn is mounted inthrust bearing 52 and in bushing 54 carried by boss 56. A spring 58 inthe aft end of the sleeve, backed up by set screw 60, presses against ashaft 62 which constantly urges the lead screw forward. The forward endof the sleeve carries an integral cone clutch member 64 to engage amating clutch member 66 on the lead' screw. The clutch members arenormally disengaged because of the pressure of spring 58 but, when thelead screw is pushed rearward, they engage to cause the sleeve and leadscrew to rotate in unison. The power for rotation is furnished by driveshaft 28 through a drive train including its drive pinion '30,intermediate driven pinion 68 and intermediate drive pinion 70, anddriven pinion 72 mounted on sleeve 50. A substantial gear reduction isincorporated because the lead screw must run much slower than the driveshaft, and any number of suitably related gears may be used.

Guide shaft 20 is connected to the lead screw by means of a nutmember.74 which may take any suitable form. It may be radially slidablymounted in a recess in the guide shaft but preferably an elongaterelatively narrow slot is formed in the guide shaft and the nut memberis in the form of an arm having its forward end mounted on pivot pin 76extending on a lateral axis so that the aft end of the arm may swing inand out radially. The aft end is provided with a formation to engagethreads 78 on the lead screw and may be a single detent but preferablyis formed as shown with threads or teeth 80 matching threads 78. Thethreads and teeth are of a form the same as or similar to the well knownbuttress type.

The forward faces 82 of the lead screw threads are set at a very sharpangle just short of perpendicular while the aft faces 84 are set at avery long angle. It will be seen that a line perpendicular to forwardfaces 82 at their point of contact with teeth 80 will pass inside pivotpin 76, with the result that axial retracting'pressure on guide shaft 20will cause a positive non-slip engagement between threads 78 and teeth80. On the other hand, advance movement of the guide shaft will causeteeth 80 to be cammed outward and the guide shaft is free to moveforward with respect to the lead screw. The nut member is heldyielding'ly in engagement with the lead screw by a spring 86 mounted ina radial passage 88 and preferably provided with a cap' 90 which makesfrictional engagement with the passage wall to resist unintentionalaxial movement of the guide shaft. However, when it is desired tocommence a drilling operation, the pressure foot may be gripped to pullthe guide shaft in a forward, or advance, direction overcoming thefriction of cap 90 and ratcheting the nut member over the threads of thelead screw.

To carry out a drilling operation, the first step is to advance theguide shaft until the contact face of the pressure pad 24 isapproximately even with the point or tip of bit 16. The drill is thenbrought into working posi-' tion and the pressure pad placed against thework piece, applying considerable axial pressure which will force theguide shaft, nut member, and lead screw aft the very short distancenecessary to cause the clutch members to engage. The motor is thenactivated to cause the bit and the lead screw to rotate. The directionof the threads is, of course, such that the screw effectively moves intothe nut member so that conversely the nut member retracts and enablesthe guide shaft and pressure foot to gradually retract while the drillbit advances relatively to penetrate the workpiece.

The bit cannot advance unless the pressure foot retracts, and thisrequires adequate axial drilling pressure. The gear reduction and thepitch of the lead screw are chosen to accomplish the desired feedrate,which is a maximum limitation rather than a minimum. The operator cannotmove the bit into the work piece any faster no matter how much force heapplies. There can be no spring back when the tip of the drill breaksthrough the rear of the work piece because the pressure is being exertedby the pressure foot which holds the work piece immobile. Thus, surge,jamming, and drill breakage are prevented. Hence the operator isrelieved of the necessity of trying to hold back just before he thinksthat the drill is about to break through. Furthermore, if the work piececomprisestwo layers, as shown, or more and particularly if they arerelatively thin and somewhat elastic, each layer tends to spring backand separate from the next layer when a bit without the present devicepenetrates it. This forms a gap into which burrs may be forced so that aproper riveted joint cannot be achieved. With the present invention, theaxial load of the pressure foot forces the layers tightly together untilthe drilling is completed. This is of par ticular value in the field ofmanufacturing aircraft structures where two or more thin sheets of metalare often overlaid for drilling in place and cannot be subsequentlyseparated to clean out burrs.

Another advantage of this construction is that the feed rate is built inand cannot be tampered with or readjusted readily by the operator. Thefeed rate may be changed intentionally in the tool crib for adjustmentto different types of work by partial disassembly and substitution ofdifferent ratio drive trains or thread pitches. A large percentage ofthe drilling jobs can be done with a feed range of two to sixthousandths of an inch per revolution of the drill bit, but any otherdesired feed rate can be provided by selectingsuitable drive reductionratios and'thread pitches. I

The present system differs from positive feed systems and is superior tothem because it provides a restricted maximum set rate which is all thata positive feed system accomplishes and it also prevents spring backetc. asexplained above, which the positive feed system does not do. i

A positive feedsystem cannot be applied to a conventional hand helddrill because there is no mounting base with a-positive reference point.However, the present invention provides this feature along with all ofits other advantages.

The use of the clutch members 64 and 66 as described above precludes thepossibility of developing backlash and allowing the drill to proceedwithout restraint if the operator should fail to follow instructions. Ifhe actuates the motor when the drill is not in working position, theclutch will be disengaged and the pressure foot will not retract;Furthermore, if the bit is partly through the work and the operatorreleases the pressure for any reason, the clutch will disengage,retraction will cease, and no backlash will develop which could undulyadvance the bit.

When a large number of identical holes are to be drilled on a productionbasis it is desirable to provide a rapid advance of the pressure footafter each operation. For this. purpose a return shaft 92 is fixedlyconnected at its forward end to pressure foot 22 and its aft portionslides axially in passage 94. Its aft end 96 is engaged by return spring98 backed up by abutment 100. The spring pressure is selected to be highenough to overcome the carnming force of teeth on threads. 78 and thefriction force of cap so that it will automatically advance the pressurefoot when axial pressure against the workpiece is removed. I

In situations where very small bits are used to drill deep holes it iscommon to back the bit out of the hole one or more times to clear thechips. When this is done with the present device in use, it is desirableto retain the pressure foot adjustment rather than using the automaticadvance so that the bit may be immediately returned to its cuttingposition. This is done by making the abutment axially movable between aforward position as shown in solid lines where the spring is compressedto an aft position shown in broken lines where the spring is unloaded.The abutment is provided with an operating pin 102 which travels in slot104 and is swung laterally to engage A special combined drill andconical reainer is frequently used to drill a hole in a work piece andthen produce a countersink for reception of a flat head fastener. Thedepth of the countersink must be very accu- 'rately controlled, andusually an elaborate stop mechanism is used for this single purpose-Inthe present device the same result is achieved by providing threads onreturn shaft 92 and mounting a threaded stop control member 108 on theshaft. This can be very accurately adjusted to give any desired depth ofhole or countersink.

While the guide shaft and pressure foot may be readily advanced bypulling on the pressure foot or by the use of the return shaft andspring 92 and 98, it is sometimes necessary or desirable to retract theguide shaft. To accomplish this, nut member 74 may be fixedly connectedto pivot pin 76 which is then rotatable in the guide shaft. One end ofthe pin is then extended laterally out of the frame or casing 18 throughslot 110 and provided with a handle 1 12 as seen in FIG. 1. The handleis turned to raise teeth 80 out of engagement with threads 78 and theguide shaft may then-be retracted.

in one of the bayonet recesses The present invention is extremelyvaluable for use in medical work such as bone drilling. Bones have hardand soft spots and it is very difficult to control the drill feed whichis critical to avoid damage to thebone structure. As schematicallyshownin FIG. 3, thesame type of device as described above is applied ona small scale to a specialized bone drill. Casing 114 contains thecontrol components and guide shaft 116 is basically the same as member20. The pressure pad 118 is preferably annular to surround bit 120 andis provided with a plurality of sharp prongs 122 spaced around themargin and extending outward so that oneor more of themwillpenetratingly engage the relatively soft bone work piece 124 andprevent lateral slippage of .the'pressure pad and the bit. An adjustablestop screw 126 mounted in a boss 128 to engage thecasing controls thedepth of penetration.

The invention may be applied to powerfeed machines, pneumatically orhydraulically driven, in the same manner as to hand held drills. In FIG.4, the casing 130 is comparable to casing 18 and contains the controlcomponents. Guide shaft 132 carries pressure foot 134 with its pressurepad 136 to engage work piece 40. A mounting base 138, adapted to besecured to a drilling fixture 140, is provided with spaced slide guidesto receive slides 142 on the sides of the casing. Feed cylinder 144,connected to base 138, is supplied with pressure fluid to force piston146 and casing 130 toward the work piece. The control operation isexactly the same as above described. An adjustable stop screw 148,carried by the casing, contacts the pressure foot to control the depthof penetration.

In FIG. 5, the mounting base 1.50 is adapted to be connected to thestructure to be drilled. For this pur pose it is provided with aclamping pin or collet 152 which passes through the work piece 40 andpressure foot 154 and into boss or flange 156 on the base to pull thelatter against the pressure foot which in turn is pulled against thework piece, engaging boss 158 and pressure pad 160. A guide rod 162 onthe pressure foot passes through ears 164 on the mounting base. Feedcylinder 166, connected to base 150, is supplied with pressure fluid toforce piston 168 and casing 130 toward the work piece. Adjustable stopscrew 170 is carried by the casing.

A stationary drill press may be equipped with exactly the same device asa hand held drill and will operate in the same way. However, themodification shown in train. Frame 18 is connected to the spindle guideby clamp 186. Guide shaft 178 extends rearward from frame 18 instead offorward but is connected to nut member 74 in the same way as previouslydescribed and is controlled in the same way.

The fixed reference member is arm 180 rigidly connected to the spindlehousing and extending laterally, with its upper surface 182 defining thereference plane. The guide shaft extends upward through or beside thearm and is provided with an adjustable contact member 184 adapted toengage surface 182. The space between the contact member and surface 182is the free travel of the drill and is so adjusted that when member 184engages surface 182 the drill tip will be just short of the work piece.When substantial downward axial pressure is now applied to the spindleguide, clutch members 64 and 66 will'engage and thedrill will bepermitted to gradually advance into the work piece.

To control the depth of penetration, an abutment 188 is provided on thesecond end portion of the guide shaft. A threaded rod 190 is mounted onthe frame and carries a threaded stop member 192 which may be adjustedto the material thickness or to the depth of a hole or countersink.'Whenthe stop member contacts the abutment, the frame cannot be loweredfarther.

This for'mv of the invention may be used with a drill press because thework piece is supported by a solid backing and ordinarily is ratherheavy stock so that the spring back problem is not encountered. However,the drill press still has the surge problem and the problem ofcontrolled penetration, both of which are eliminated by the use of thisinvention. 1

The feed control described above serves to limit the maximum rate ofadvance of a drill bit but not the mini- .mum rate. Hence it differsfrom the positive feed type which forces a drill bit to advanceregardless of the nature of a particular work piece. The-clutch faces ofthe present device are preferably metallic, such as bronze againststeel, and some slippage occurs when pressures are low. The operator mayapply just enough pressure to cause the feed screw to rotate slowly andproduce a feed rate of the order of 0.001 inch per revolution of thebit, or any greater pressure up to that which permits no slippage andthe maximum feed rate determined by the gear ratio of the drive train.It has been found that feed rates controlled by selected pressuresproduce superior holes and greatly improve cutting tool life.

The detent means 74, has been shown and described simply as a nut memberfor simplicity of illustration. However, it will be apparent that it maytake various other forms such as a split nut or a slider with aratcheting tooth. In cases where loads are high, as in power feeds, thelead screw may take the form of a sleeve with internal threads and thedetent means may be an elongate shaft with a plurality of threadedsegments resiliently biased radially outward to engage the threads ofthe sleeve and act in the same way as the exemplary form shown in thedrawings. 7

A further modification of the devices of FIGS. 2 and 3 is schematicallyillustrated in FIG. 7. This form is particularly useful for surgicalwork and provides positive control of the entire drill unit to insuredrilling holes in the human skull without danger of damaging the brainby surging of the drill point after breakthrough. Drill motor 194 may beof any suitable type, and frame 196 may be attached to it by a screwthread connection 198 as shown. The forward end of drive spindle 200 ismounted in bearing 202 and is provided with the element 204 of a conetype clutch.

Drive shaft 206 is slidably mounted in bearings 208 and hasaconventional chuck 210 at its forward end to grip the drill bit 212. Atits aft end, it carries the other element 214 of the cone type clutch,and the shaft is biased forward by spring 216 located between theelements. It will be apparent that the spring may be located at anypoint where it will perform the desired function. If the motor isactivated without pressure on the drill, the spindle will rotate freelybut will not rotate the drive shaft. However, when the point 218 of thedrill bit contacts the work piece 220, which is a human skull, the axialpressure will retract the drive shaft and cause engagement of the clutchelements, resulting in rotation of the bit. When the bit breaks throughthe rear surface of the skull, the axial pressure drops off and may evenbecome tension because of the'advancing of the drill flutes. The clutchelements immediately disengage and drill rotation ceases. However, withno further preventive action, the drill may be pushed forward withoutrotation and cause serious damage to the brain.

This danger is overcome by provision of the feed control including guideshaft 20, detent means 74, and lead screw 42. The latter is provided asbefore with a forward shank 44 mounted in bearing 46 and an aft shank 48mounted in sleeve 50 which in turn is mounted in a bearing 222. Spring224 presses against the aft end'of shank 48 to urge the lead screwforward. Drive shaft 206 carries a fixedly mounted drive gear 226 whichengages driven gear 228 fixed to sleeve 50 which also carries a clutchelement 230 to engage clutch element 232 secured to the aft end of thelead screw.

In the operation of this modification, the pressure foot is adjusted sothat when point 218 just engages the skull the prongs 122 will beslightly spaced, preferably even less than indicated in the drawing. Assoon as the drill begins to penetrate, one or more prongs 122 willengage and cause retraction of guide shaft 20 and lead screw 42. Thisengages clutch members 230 and 232 to activate the lead screw andgradually retract the pressure foot 1 18 with respect to the frame.Drilling continues until point 218 breaks through the inner wall. Atthis point, the axial pressure on the drill drops off and clutch members204 and 214 disengage, stopping the drill immediately. Since the leadscrew is-controlled by the drive shaft through gears 226 and 228, italso stops immediately and prevents any inward movement of the drill,thus'avoiding any possibility of damage to the brain. It will be notedthat if the lead screw were controlled by spindle 200 instead of byshaft 206 it would continue to feed the drill forward with resultingdamage even thoughthe drill would not be rotating. Consequently, it willbe seen that the combination described provides total protection fordelicate surgical procedures.

The form of FIG. 7 also has applications to some manufacturingoperations. In cases where a built-up structure has two elements whichare closely spaced depthwise and it is necessary to drill a hole in oneele ment without cutting or scratching the other, the feed control willoperate in the same way as just described and provide completeprotection for the second element.

I claim:

1. A feed control for use in combination with a drill motor having adrill shaft driving a drill bit, comprising:

a frame having means for attachment to the drill motor;

a guide shaft having an aftportion axially slidably mounted on the framefor'axial extension and retraction parallel to the axis of the drillmotor;

a pressure foot fixedly mounted on the forward free end of the guideshaft and having a pressurepad to pressurally engage a work pieceadjacent to the point of engagement of the drill bit;

and restraining means to positively oppose retraction of the guide shaftunder axial pressure,

the restraining means being controlled in position by the rotation ofthe drill motor drive shaft to enable the guide shaft and foot togradually retract with respect to the frame in response to axialpressure between the foot and the work piece and produce a correspondingadvance of the drill bit with respect to the pressure foot.

2. A feed control as claimed in claim 1;

the restrainingfmeans including a resiliently biased releasable detentconnection to permit rapid extension of the guide shaft.

3. A feed control as claimed in claim 1;

and adjustable stop means to limit retraction of the guide shaft andpressure foot to a predetermined extent and consequently limit thepenetration of the drill bit into the workpiece.

4. A feedcontrol for use in combination with a drill motor having adrive shaft driving drill bit, comprising;

a frame having means for attachment to the drill motor;

a guide shaft having an aft portionaxially slidably mounted on the framefor extension and retraction movement parallel to the axis of the drillmotor;

a pressure foot fixedly mounted on the forward free end of theguideshaft and having a pressure pad to pressurally engage a work pieceadjacent to the point of engagement of the drill bit; I

a lead screw mounted on the frame for rotation about an axis parallel tothe axis of the drill motor;

drive means connected between the lead screw and the drive shaft of thedrill motor tocause rotation the rotation of the lead screw enablingthedetent means and the guide shaft to gradually retract with respect tothe frame in response to axial pressure and effectively enable the drillbit to advance with respect to the work piece at a. limited feed rate.

5. A feed control as claimed in claim 4;

the detent means being resiliently biased into engagement with the leadscrew to maintain contact for application of axial pressure, and readilyreleasable against the biasing force in the absence of axial pressure topermit rapid extension and retraction of the guide shaft.

6. A feed control as claimed in claim 5;

the guide shaft being provided with an axially elongate radiallyextending cavity;

the detent means being in theform of an elongate arm located in theguide shaft cavity and being pivotally mounted at its forward end in theforward portion of the cavity and provided at its aft end with aformation for pressurally engaging the threads of the lead screw.

7. A feed control as claimed in claim 6;

the pivotal mountingof the arm comprising a pivot shaft rotatablymounted in the guide shaft on an axis perpendicular to the axis of theguide shaft;

the arm being fixedly connected to the pivot shaft for rotationtherewith; v i

and the pivot shaft having one endextendin g outward of the frameandprovided with a formation to be manually gripped for rotation of thepivot shaft and arm.

8. A feed control as claimed in claim 4 and adjustable stop meansinterengaging between the pressure foot and the frame to positivelylimit the extent of retraction of the pressure foot.

9. A feed control as claimed in claim 4;

the detent means being resiliently biased into engagement with the leadscrew to maintaincontact for application of axial pressure, and readilyreleasable against the biasing force in the absence of axial retractingpressure; a

and spring means operable to urge the guide shaft toward fully extendedposition in the absence of axial retracting pressure.

10. A feed control as claimed in claim 9;

an axial passage in the frame extending parallel to the axis of theguide shaft;

a return shaft having a forward end connected to the pressure foot andan aft portion slidably extending into the passage;

and a return spring mounted in the passage and contacting the aft end ofthe return shaft to urge it forward and move the guide shaft andpressure foot toward extended position.

11. A feed control as claimed in claim 10;

and an abutment axially movable in the pas'sage'toa forward positionengaging theaft end of the spring to place it under compression and toan aft position to unload the spring.

12. A feed control as claimed in claim 10;

the return shaft being threaded;

and a stop member threaded on the return shaft and axially adjustable topredetermined positions to engage the frame and limit the retraction ofthe pressure foot.

13. A feed control as claimed in claim 4;

a spring-loaded shoe on the guide shaft frictionally engaging the frameto prevent unintentional axial movement of the guide shaft in theabsence of axial pressure on the work piece.

14. A feed control as claimed in claim 4;

the drive means and the lead screw being provided with normally spaced,relatively axially movable, cooperating clutch members;

the lead screw being axially movable to a limited extent with the guideshaft and nut member in response to axial retraction pressure fromengagement with the work piece to engage the clutch members and causerotation of the lead screw in response to rotation of the drill motordrive shaft.

15. A feed control as claimed in claim 14;

' and spring means operative to urge the lead screw axially to aposition to disengage the'clutch members when axial pressure on the workpiece is released.

16. A feed control as claimed in claim 4;

the drive means including a plurality of interengaging driving anddriven gears;

an extension shaft mounted at its aft end on the drill motor drive shaftand carrying a drill bit chuck at its forward end;

and the first driving gear of the drive means being fixedly mounted onthe extension shaft for rotation therewith. r 4

17. A feed control as claimed in claim 4;

the pressure pad being provided with a plurality of spaced sharp prongsextending from its work face to penetratingly engage a soft work pieceand prevent lateral slippage.

18. A feed control as claimed in claim 4;

further including a supporting base adapted to be mounted on a workholding fixture;

the frame andan attached drill motor being slidably mounted on the basefor axial movement toward and away from a work piece to be drilled;

and fluid power means to yieldably force the frame and drill motortoward the work piece.

19. A feed control as claimed in claim 4;

further including a supporting base provided with clamping means forrigidly connecting it to a work piece assembly;

the framev and an attached drill motor being slidably mounted on thebase for axial movement toward and away from the assembly;

and fluid power means to yieldably force the frame and drill motortoward the assembly.

20. A feed control for use in combination with a drill motor having adrive spindle, an axially movable drive shaft for driving a drill bit,and clutch means between the drive spindle and drive shaft engageable byrelative retraction of the drive shaft, comprising:

a frame having means for attachment to the drill motor;

a guide shaft having an aft portion slidably mounted on the frame foraxial extension and retraction parallel to the axis of the drive shaft;

a pressure foot fixedly mounted on the forward free end of the guideshaft and having a pressure pad to pressurally engage a work pieceadjacent to the point of engagement of the drill bit;

and restraining means to positively oppose retraction of the guide shaftunder axial pressure;

the restraining means being controlled in position by the rotation ofthe drive shaft to enable the guide shaft and foot to gradually retractwith respect to the frame in response to axial pressurebetween the footand the work piece and produce a corresponding advance of the drill bitwith respect to the pressure foot; y

the drive shaft being driven by the drive spindle only throughengagement of the clutch means in response to retraction of the driveshaft under axial pressure;

the clutch means being releasable in response to cessation of such axialpressure to discontinue both rotation of the drive shaft and drill bitand retraction of the pressure foot.

21. A feed control as claimed in claim 20;

and spring means arrangedto bias the clutch means to normally disengagedcondition.

22. A feed control as claimed in claim 20;

the restraining means including second clutch means biased to normallydisengaged condition to immobilize the restraining means and engageablein response to axial pressure on the pressure foot to activate therestraining means for retraction of the guide shaft.

23. A feed control for use in combination with a drill 40 motor having adrive shaft driving a drill bit, comprising:

a frame having means for attachment to the drill motor;

a guide shaft having an aft portion axially slidably mounted on theframe for axial extension and retraction parallel to the axis of thedrill motor;

a pressure foot fixedly mounted on the forward free end of the guideshaft and having a pressure pad to pressurally engage a work pieceadjacent to the point of engagement of the drill bit;

and restraining means to positively oppose retraction of the guide shaftunder axial pressure,

the restraining means being controlled in position in a timed relationwith the rate of rotation of the drill motor shaft to enable the guideshaft and foot to gradually retract with respect to the frame inresponse to axial pressure between the foot and the work piece andproduce a corresponding advance of the drill bit with respect to thepressure foot.

1. A feed control for use in combination with a drill motor having adrill shaft driving a drill bit, comprising: a frame having means forattachment to the drill motor; a guide shaft having an aft portionaxially slidably mounted on the frame for axial extension and retractionparallel to the axis of the drill motor; a pressure foot fixedly mountedon the forward free end of the guide shaft and having a pressure pad topressurally engage a work piece adjacent to the point of engagement ofthe drill bit; and restraining means to positively oppose retraction ofthe guide shaft under axial pressure, the restraining means beingcontrolled in position by the rotation of the drill motor drive shaft toenable the guide shaft and foot to gradually retract with respect to theframe in response to axial pressure between the foot and the work pieceand produce a corresponding advance of the drill bit with respect to thepressure foot.
 2. A feed control as claimed in claim 1; the restrainingmeans including a resiliently biased releasable detent connection topermit rapid extension of the guide shaft.
 3. A feed control as claimedin claim 1; and adjustable stop means to limit retraction of the guideshaft and pressure foot to a predetermined extent and consequently limitthe penetration of the drill bit into the work piece.
 4. A feed controlfor use in combination with a drill motor having a drive shaft drivingdrill bit, comprising; a frame having means for attachment to the drillmotor; a guide shaft having an aft portion axially slidably mounted onthe frame for extension and retraction movement parallel to the axis ofthe drill motor; a preSsure foot fixedly mounted on the forward free endof the guide shaft and having a pressure pad to pressurally engage awork piece adjacent to the point of engagement of the drill bit; a leadscrew mounted on the frame for rotation about an axis parallel to theaxis of the drill motor; drive means connected between the lead screwand the drive shaft of the drill motor to cause rotation only of thelead screw in response to the rotation of the drive shaft and axialpressure on the pressure foot; and detent means connected to the guideshaft and engaging the threads of the lead screw to restrain retractionof the guide shaft under axial pressure; the rotation of the lead screwenabling the detent means and the guide shaft to gradually retract withrespect to the frame in response to axial pressure and effectivelyenable the drill bit to advance with respect to the work piece at alimited feed rate.
 5. A feed control as claimed in claim 4; the detentmeans being resiliently biased into engagement with the lead screw tomaintain contact for application of axial pressure, and readilyreleasable against the biasing force in the absence of axial pressure topermit rapid extension and retraction of the guide shaft.
 6. A feedcontrol as claimed in claim 5; the guide shaft being provided with anaxially elongate radially extending cavity; the detent means being inthe form of an elongate arm located in the guide shaft cavity and beingpivotally mounted at its forward end in the forward portion of thecavity and provided at its aft end with a formation for pressurallyengaging the threads of the lead screw.
 7. A feed control as claimed inclaim 6; the pivotal mounting of the arm comprising a pivot shaftrotatably mounted in the guide shaft on an axis perpendicular to theaxis of the guide shaft; the arm being fixedly connected to the pivotshaft for rotation therewith; and the pivot shaft having one endextending outward of the frame and provided with a formation to bemanually gripped for rotation of the pivot shaft and arm.
 8. A feedcontrol as claimed in claim 4; and adjustable stop means interengagingbetween the pressure foot and the frame to positively limit the extentof retraction of the pressure foot.
 9. A feed control as claimed inclaim 4; the detent means being resiliently biased into engagement withthe lead screw to maintain contact for application of axial pressure,and readily releasable against the biasing force in the absence of axialretracting pressure; and spring means operable to urge the guide shafttoward fully extended position in the absence of axial retractingpressure.
 10. A feed control as claimed in claim 9; an axial passage inthe frame extending parallel to the axis of the guide shaft; a returnshaft having a forward end connected to the pressure foot and an aftportion slidably extending into the passage; and a return spring mountedin the passage and contacting the aft end of the return shaft to urge itforward and move the guide shaft and pressure foot toward extendedposition.
 11. A feed control as claimed in claim 10; and an abutmentaxially movable in the passage to a forward position engaging the aftend of the spring to place it under compression and to an aft positionto unload the spring.
 12. A feed control as claimed in claim 10; thereturn shaft being threaded; and a stop member threaded on the returnshaft and axially adjustable to predetermined positions to engage theframe and limit the retraction of the pressure foot.
 13. A feed controlas claimed in claim 4; a spring-loaded shoe on the guide shaftfrictionally engaging the frame to prevent unintentional axial movementof the guide shaft in the absence of axial pressure on the work piece.14. A feed control as claimed in claim 4; the drive means and the leadscrew being provided with normally spaced, relatively axially movable,cooperating clutch members; thE lead screw being axially movable to alimited extent with the guide shaft and nut member in response to axialretraction pressure from engagement with the work piece to engage theclutch members and cause rotation of the lead screw in response torotation of the drill motor drive shaft.
 15. A feed control as claimedin claim 14; and spring means operative to urge the lead screw axiallyto a position to disengage the clutch members when axial pressure on thework piece is released.
 16. A feed control as claimed in claim 4; thedrive means including a plurality of interengaging driving and drivengears; an extension shaft mounted at its aft end on the drill motordrive shaft and carrying a drill bit chuck at its forward end; and thefirst driving gear of the drive means being fixedly mounted on theextension shaft for rotation therewith.
 17. A feed control as claimed inclaim 4; the pressure pad being provided with a plurality of spacedsharp prongs extending from its work face to penetratingly engage a softwork piece and prevent lateral slippage.
 18. A feed control as claimedin claim 4; further including a supporting base adapted to be mounted ona work holding fixture; the frame and an attached drill motor beingslidably mounted on the base for axial movement toward and away from awork piece to be drilled; and fluid power means to yieldably force theframe and drill motor toward the work piece.
 19. A feed control asclaimed in claim 4; further including a supporting base provided withclamping means for rigidly connecting it to a work piece assembly; theframe and an attached drill motor being slidably mounted on the base foraxial movement toward and away from the assembly; and fluid power meansto yieldably force the frame and drill motor toward the assembly.
 20. Afeed control for use in combination with a drill motor having a drivespindle, an axially movable drive shaft for driving a drill bit, andclutch means between the drive spindle and drive shaft engageable byrelative retraction of the drive shaft, comprising: a frame having meansfor attachment to the drill motor; a guide shaft having an aft portionslidably mounted on the frame for axial extension and retractionparallel to the axis of the drive shaft; a pressure foot fixedly mountedon the forward free end of the guide shaft and having a pressure pad topressurally engage a work piece adjacent to the point of engagement ofthe drill bit; and restraining means to positively oppose retraction ofthe guide shaft under axial pressure; the restraining means beingcontrolled in position by the rotation of the drive shaft to enable theguide shaft and foot to gradually retract with respect to the frame inresponse to axial pressure between the foot and the work piece andproduce a corresponding advance of the drill bit with respect to thepressure foot; the drive shaft being driven by the drive spindle onlythrough engagement of the clutch means in response to retraction of thedrive shaft under axial pressure; the clutch means being releasable inresponse to cessation of such axial pressure to discontinue bothrotation of the drive shaft and drill bit and retraction of the pressurefoot.
 21. A feed control as claimed in claim 20; and spring meansarranged to bias the clutch means to normally disengaged condition. 22.A feed control as claimed in claim 20; the restraining means includingsecond clutch means biased to normally disengaged condition toimmobilize the restraining means and engageable in response to axialpressure on the pressure foot to activate the restraining means forretraction of the guide shaft.
 23. A feed control for use in combinationwith a drill motor having a drive shaft driving a drill bit, comprising:a frame having means for attachment to the drill motor; a guide shafthaving an aft portion axially slidably mounted on the frame for axialextensIon and retraction parallel to the axis of the drill motor; apressure foot fixedly mounted on the forward free end of the guide shaftand having a pressure pad to pressurally engage a work piece adjacent tothe point of engagement of the drill bit; and restraining means topositively oppose retraction of the guide shaft under axial pressure,the restraining means being controlled in position in a timed relationwith the rate of rotation of the drill motor shaft to enable the guideshaft and foot to gradually retract with respect to the frame inresponse to axial pressure between the foot and the work piece andproduce a corresponding advance of the drill bit with respect to thepressure foot.